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Thursday, August 24, 2017

Why we’re here (geology is destiny)

[In 1881, an unhappy Bill Nye quit his job with the Laramie Daily Sentinel—the first and only paper in town—and started a competitor, naming it the Laramie Boomerang after his mule, who “always came back.” Sure enough, while the Sentinel folded in 1895, the Boomerang still shows up daily. One of its more popular features is the monthly column, Laramie’s Living History. I’ve contributed several articles, about the role of botany and geology in local history. Here’s the most recent (with a few additional photos).]

[August 12, 2017]Anyone who has lived in Laramie long knows why we’re here: 150 years ago, the Union Pacific (UP) passed through the Laramie Valley (southeast Wyoming) during construction of the nation’s first transcontinental railroad. But that’s only part of the story. The full explanation begins much earlier, back when a fortuitous combination of mountain-building and erosion created an easy route over the Laramie Mountains—the Gangplank.

One hundred million years ago, much of Wyoming was underwater, covered by the Western Interior Seaway, which split the continent from north to south. But it was not to last. Thirty million years later the land began to rise, forcing the sea to retreat. This was when the Laramie Mountains were born, part of a long and widespread episode of mountain-building that created most of the Rockies.

For the next 10-15 million years the Laramie Mountains rose, while at the same time erosion wore them down. Such is the fate of mountains—as soon as uplift starts, erosion sets in. As much as 20,000 feet of material is thought to have been removed from the crest of the range. Streams and wind carried down the debris, depositing it on the flanks and eventually burying the Laramie Mountains in their own rubble.

It’s hard to imagine such huge changes: a sea disappearing, mountains rising, mountains worn down and buried.But at the scale of geologic time, the Earth’s surface is dynamic, always changing. This we know, though we can’t always explain why. Many mysteries remain … for example, today’s Laramie Mountains.

Ten million years ago, when the range was mostly buried beneath a thick covering of sedimentary rocks, the east flank was a broad gently-sloping plain. That’s not the case today (think Sybille Canyon!); obviously something happened. We know that erosion resumed, but why? Perhaps the entire region was uplifted, invigorating streams. Maybe climate change brought greater precipitation. Whatever the reason, the sedimentary cover was removed during what geologists call the Great Exhumation. The Laramie Mountains were disinterred … almost.

Due to a serendipitous evolution of topography, the Great Exhumation left intact a gently-sloping wedge of sedimentary rocks in the southeast part of the range. Several million years later, on September 21, 1865, General Grenville M. Dodge, soon-to-be chief engineer of the UP, scrambled up the ridge south of Crow Creek and found himself on this wedge. He was ecstatic. Here was an easy route up the Laramie Mountains, later christened “the Gangplank.”

Looking east along the Gangplank, traversed by I-80 and the UP Railroad; added star marks contact of ancient granite in foreground and much younger sedimentary rocks behind. RD Miller photo (USGS).

Just two years after Dodge’s lucky find, on the evening of October 21, 1867, UP tracklayers entered today’s Wyoming. They barely paused at Cheyenne, pushed west up the Gangplank, wound down into the Laramie Valley, and in early May of 1868, reached Laramie City, where eager new residents had been throwing up buildings the week before. Then the tracklayers raced west, laying another 350 miles of track and establishing numerous “instant towns” before leaving Wyoming in early 1869.

The Gangplank is most striking at its western tip where the younger sedimentary rocks meet the ancient Sherman Granite. Only a few hundred feet separate the drainages of Crow and Lone Tree Creeks, and, though imperceptible to us, these streams are hard at work. Every year they erode a bit more rock and soil, sending it down to the North Platte River, the Missouri, and eventually the Gulf of Mexico. In other words, the Gangplank won’t be here forever. You might want to take a look at it while you still can!

From Laramie, follow Interstate 80 east to the Summit and the Sherman Granite, which forms the crest of the range. This is ancient rock—formed from magma 1.7 billion years ago. It was exposed much later during uplift of the Laramie Mountains. I-80 stays on Sherman Granite past Blair, Vedauwoo, Tree-in-the-Rock, and Buford. Here the terrain is relatively flat, and it’s tempting to think this is the Gangplank. But road cuts and occasional blobs of granite show otherwise.

Continue past Remount Road (Exit 339) and Harriman Road (Exit 342, large quarry). Just a short distance further, before the Wyoming Department of Transportation sand/salt storage structure, is the transition to the Gangplank. But don’t slow down!! I-80 traffic is too heavy and fast for gawking. Instead, continue about three miles to Exit 345; drive to the truck parking area on the Gangplank.

At the east end of the parking area is a sign illustrating the Gangplank rising from the Great Plains onto the Laramie Mountains. The text makes more sense when you realize the sign originally stood at the west end of the Gangplank (you’ll be there soon). The slope next to the parking area reveals some of the sedimentary rocks that covered the range ten million years ago, before the Great Exhumation.

After mastering the diagram on the sign, find the westbound frontage road at the opposite end of the parking area. Set your trip meter to zero and drive the Gangplank. At 2.0 miles, just past Mile Marker 2 (green) and near the top of the hill, you will leave the ten-million-year-old sedimentary rocks and return to the ancient Sherman Granite. Here, at the tip of the Gangplank, you can straddle more than a billion years of Earth history.

From the hilltop, the view east toward Cheyenne gives a good feel for how narrow this end of the Gangplank is. The north side (left) drops steeply down to the South Fork of Crow Creek. Lone Tree Creek is off to the south (not visible here). In the distance, the Gangplank widens and merges with the Great Plains in Nebraska.

Looking east from the west end of the Gangplank; South Fork Crow Creek on left.

Editor’s Note:This is one in a series written for the Albany County Museum Coalition that promotes interest in local cultural and natural history.Hollis Marriott came to Wyoming in 1977 to work at Devils Tower National Monument, fell in love with the wildness of the state, and stayed. She received a master’s degree in botany from UW in 1985. Now retired, she indulges her passions for botany, geology, and the great outdoors in general.

[Blogger’s Note: It’s always a pleasure to write for the Boomerang, for I’m guaranteed an enthusiastic audience. Many residents rightfully take a great interest in local history, both human and natural. Thanks to the following for help with content and editing: Judy Knight, editor of Laramie's Living History; Albany County Railroad Historian Jerry Hansen; botanist and railroad buff Dennis Knight; and Mike Nelson of CSMS Geology Post(see link for more about Gangplank geology).